1. Priority Information
This application claims priority from European Patent Application No. 03077016.8, filed Jun. 27, 2003, herein incorporated by reference in its entirety.
2. Field of the Invention
The present invention relates to an apparatus for positioning a substrate, a method for positioning a substrate relative to a substrate table, and an associated device manufacturing method.
3. Description of the Related Art
Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may be used to generate a desired circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist).
The term “patterning device” as here employed should be broadly interpreted as referring to a device that can be used to impart an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning devices include:                a mask: the concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmission mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table/holder/holder, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        a programmable mirror array: one example of such a device is a matrix-addressable surface having a visco-elastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as non-diffracted light. Using an appropriate filter, the non-diffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation mechanism. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described here above, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        a programmable LCD array: an example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table/holder/holder; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth here above.
In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table/holder/holder, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table/holder parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table/holder is scanned will be a factor M times that at which the mask table/holder/holder is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate table/holders (and/or two or more mask table/holders). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
During the lithographic process, a substrate is often subjected to multiple exposures or sets of exposures. In between successive exposures or sets of exposures, the substrate is taken from the substrate table and typically out of the lithographic apparatus to undergo further processing, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features, as already stated above. After such processing, the substrate is often transported back to the substrate table for a next exposure or set of exposures. The result of such a procedure is, for example, multiple layers on a substrate.
The substrate is transported to and from the substrate table using, for instance, a robot arm. The robot arm is arranged to grip a substrate at a certain position, move the substrate, and release the substrate at another position. When the robot arm hands over the substrate to the substrate table, the substrate should be positioned with great accuracy. Therefore, the position (rotation and/or translation) of the substrate with respect to the robot arm needs to be accurately determined.
The determination of the position of the substrate relative to the robot arm is typically done with the use of a pre-aligner. The pre-aligner measures the substrate position and the substrate is positioned with respect to the robot arm for transport to the substrate table. Thus, the pre-aligner ensures that the relative position of the substrate with respect to the robot arm is known, so that the robot arm can accurately deliver the substrate to the substrate table.
At the substrate table, the robot arm typically provides the substrate to one or more pins of the substrate table. The pins of the substrate table displace the substrate to the supporting surface of the substrate table. Consequently, the positional accuracy of the substrate on the substrate table surface are limited by the measuring accuracy of the pre-aligner and the hand-over by the robot arm to the pin(s) and by the pin(s) to the substrate table surface.
After the substrate is positioned on the substrate table surface, the positional error (translational, rotational) of the substrate orientation relative to the substrate table orientation is determined. This is typically done, for instance, by using an alignment system to measure the position of two marks provided on the substrate and two marks provided on the substrate table or a chuck, on which the substrate is supported. If the marks are out of the range of the alignment system, the substrate is removed from the substrate table and optionally replaced on the substrate table after a further pre-alignment. If the marks are within the measurement range of the alignment system, the positional error between the orientation of the substrate and the orientation of the substrate table is measured and determined. This positional error is used by the substrate table/chuck positioning and position measurement means to ensure that the patterned beam is projected correctly onto the substrate.
In some circumstances, the substrate table/chuck is rotated to adjust for the above-referenced positional error in order that the patterned beam is correctly projected onto the substrate. However, it will be understood by a person skilled in the art that a rotation of the substrate table/chuck can introduce errors in the measurement performed by an interferometer position measurement means, since the surface of a mirror mounted on the substrate table/chuck and used by the interferometer position measurement means may no longer be perpendicularly positioned with respect to the interferometer position measurement means. As will be understood, this can lead to overlay errors where the positional error is different between an exposure or set of exposures and a subsequent exposure or set of exposures.
Furthermore, despite the pre-alignment and hand-over described above, the positioning of the substrate on the substrate table may not be exactly the same for a next exposure or set of exposures, since the position of the substrate with respect to a support structure (e.g. a pimple pattern) on the substrate table may be different for the next exposure or set of exposures due to inaccuracies of the pre-aligner and inaccurate substrate-handling processes. Such differing positions can lead to overlay errors. For example, a local inaccuracy in the support structure of the substrate table could cause a local deformation in the top surface of the substrate. If the position of the substrate with respect to the support structure is different for a next exposure or set of exposures, the deformation in the top surface of the substrate may be different for such next exposure or set of exposures. This could possibly lead to overlay errors as will be understood by a person skilled in the art.